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Патент USA US3034357

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May 15, 1962
J. E. STARR
3,034,347
STRAIN GAUGE BRIDGE CIRCUIT ARRANGEMENT, ‘PARTICULARLY
FOR LOAD CELLS
Filed Dec. 1, 1960
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OUTPUT
INVENTOR.
JamesE' S‘carr.
_
BY MLQJQQMK
ATTORNEY
g?di?i?
,.
United States Patent
” ICC'
Patented May 15, 1962
1
2
3,034,347
The above and additional objects and advantages are
secured, according to an illustrated embodiment of this
invention applied to a load cell spring element, by a bridge
network having four substantially similar resistance
STRAEN GAUGE BRIDGE CRCUIT
GE
MENT, PARTICULARLY FOR LOAD CELLS
James E. Starr, Cumberland, Md, assignor to The Budd
Company, Philadelphia, Pa, a corporation of Penn
sylvania
Filed Dec. 1, 1960, Ser'. No. 72389
2 Claims. (Cl. 73-141)
strain gauges bonded to the spring element and connected
respectively in the four bridge arms, a temperature de
pendence correction resistor and a balance point correc
tion resistor inserted respectively between adjacent bridge
This invention pertains to an improved bridge circuit
of the type having four arms each comprised of a strain
gauge exhibiting resistance variations in accordance with
imposed strain variations. More particularly, this inven
arms at opposite ends of one bridge diagonal, ?rst and
second remote shunting resistors each connected in par
allel with one of the correcting resistors and each having
a variable'midtap, ?rst external circuit means connected
between said midtaps, and second external means con
nected across the other bridge diagonal, one of the ex
tion pertains to improvements in such a strain gauge,‘
bridge network adapted for application to a load cell 15 ternal circuit means generating a bridge excitation input
and the other external circuit means generating an output
spring element where the bridge output is required to be
responsive to .bridge unbalances related to spring element
an accurate and reproducible function of external load
load strains.
ing forces imposed upon the spring element.
The features of this invention believed to be novel are
The conventional four-arm strain gauge bridge is theo
retically capable of maintaining an exact balance inde 20 distinctly pointed out in the appended claims; however, a
better understanding of the invention will be had upon
pendently of ambient temperature provided that all four
consideration of the following speci?cation taken in con
arms act in an identical'manner in response to tempera
junction with the accompanying drawing wherein:
ture variations. In practice, however, it is never possible
FIG. 1 illustrates application of the bridge circuit of
to approach this condition except by empirical adjust
ments of a given strain gauge bridge circuit after its 25 this invention to a representative load cell;
PEG. 2 is a schematic illustration of the impedance
application to a speci?c structure. Even if the several
elements of the ‘bridge network of this invention;
strain gauges were identical, some initial bridge unbal
FIG. 3 is an arti?cial perspective view of the orienta
ance and some bridge temperature dependence would re
tion of the bridge network impedances on the gauged
sult from differences in the connections which must be
made from strain gauge to strain gauge in order to form 30 strain surface of the spring element of the FIG. 1 load
the bridge network. The presence of solder joints, of
cell.
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The load cell 10 of FIG. 1 comprises a spring element
‘12 generally cylindrical about a vertical load axis and
apertured normally of that axis to de?ne a cylindrical
lead wires, and the like, always leave some residual tem
perature dependence in the completed circuit. Further, 35 gauged-strain surface v14. Upper and. lower loading
members 16 and 18 and load concentrating elements 20
variations in resistance, resistivity, and strain resistance
and 22 are aligned coaxially with spring element 12.
coeflicients for the bridge inter-connections can always be
slightly unequal lengths of connecting lead wires, of 1m—
avoidable changes in temperature coe?icients along the
expected to result in a residual bridge unbalance even
when no external strain is imposed upon any of the bridge
strain gauges.
Conventional strain gauges of the bonded and resistance
vfoil type T, B, L, and R are adhesively attached to gauged
40 strain surface 14 respectively at diametricallyopposcd po
'
sitions along and ‘at right angles to the load axis. ‘It is
A common method for correcting residual temperature I
assumed that loadcell spring element 12 is properly pro
dependence is to insert a small resistor having a high tem
portioned so that all four gauges sense substantially equal
perature coe?icient in one of the bridge arms. In order
load and temperature strains. However, temperature
to determine the required characteristics of the tempera
ture correcting resistor and its proper placement in the 45 strains are of the same sign while load strains sensed by
gauges T and B are opposite in sign to those sensed by
bridge, it is necessary to check the performance of the
gauges L and R.
completed bridge versus temperature, to adjust the con
A housing 24 isprovided for isolation of the active load‘
dition of the temperature correcting resistance, and then to
cell elements from ambient conditions and may comprise
recheck bridge response to ambient temperature varia
tions. Whenever such a temperature correcting resistor 50 a cylinder 26 closed at the bottom by disk 28 extending
between and attached to cylinder 26 and loading member
is inserted in a bridge circuit, the balance point of the
18. Axial displacement of elements 16 and 13 is de
bridge will be disturbed and it is usually necessary to
correct this by inserting and adjusting a second resistance . coupled from the housing 24 by employing an annular
bellows 30 as the upper housing closure extending be
which has a negligible temperature dependence in order
to rebalance the bridge. It is usually necessary to repeat 55 tween and attached to cylinder 26 and element 16. Lead
wires, not shown, from the internal bridge circuitry may
these several adjustments because the temperature de
be led out from housing 20 through aperture 32. After
pendence and bridge balance corrections interact. Bridge
the desired internal atmosphere is provided for the spring
operations are affected not only by variations in bridge
element environment, aperture 32 may be sealed by any
components but also by variations in their environment;
convenient means.
i‘
Therefore, a load cell bridge circuit should be assembled
The above is what would be considered a preferred
on its spring element and hermetically sealed within its
design in the load cell art if it were not for the difficulty
protecting enclosure during bridge adjustment procedures
of making bridge correction adjustments. It has been
so that the operational environment is present during
necessary, however, to utilize temporary closure means
rebalancing. However, since each adjustment required
access to the bridge, it was‘ usually necessary to remove 65 for the housing in order to allow disassembly and reas
sembly before and during correction procedures. Ac
the enclosure for insertion or ‘alteration of the correcting
cording to this invention, however, additional correcting
resistances, an obviously cumbersome .and expensive
resistors 34 and 36 are included within the internal‘bridge
circuit and cooperate with certain external elements so
Therefore, it is an object of this invention to provide
an improved strain gauge bridge circuit allowing bridge 70 that bridge correction procedures may be carried out after
hermetic sealing of the spring element environment. The
temperature dependence and initial bridge balance errors
operation.
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to be corrected from a remote location.
.
external bridge circuit elements, shuntingresistors 38 and
3,034,347
3
. 40, are housed in an appended junction box 412 having a‘
it is preferable that balance correction resistor 36 be larger -
V readily removable cover plate 44.‘
than temperature correction resistor 34 so that sufficient
range will always be available for rebalancing after cor
-
The bridge circuit internal and external elements are ‘
shown schematically in FIG; 2. ' Strain gauges T, L, B,
and R are connected in series in that order in'aocordance
rection for temperature dependence.
FIG. 3 is an arti?cial perspective View of gauging sur
face 14 of the FIG.’ 1 load cell, as it would appear if that
with the well known Wheatstone bridge principle that
bridge unbalance is proportional to di?erence of resistance
surface could be stretched laterally at circular edge 58
changes in any two adjacent arms'or proportional to sum~
mation of resistance changes in any two opposite arms.
According to this invention,'however, correcting resistors
until both edges 58 and 69 were in the same lateral plane.
Strain gauges T, L, B, and R, are located ‘at their respec
10
1 '34 and 36 are respectively serially inserted between ad
jacent-arm strain gauges at opposite ends of one or" the
tive angular positions on the projection. Longitudinal
positions within the load cell spring element aperture are
indicated by the radial positions of the gauges between
two ‘bridge diagonals and shunting resistors 38 and 443
edges 58 and 6t).
Correcting resistors 34 and’ 36 are
are connected, respectively, in parallel with the correcting
located on surface 14 at the 45 ‘’ low strain positions which
resistors. Variable midtap connections 46 and 48, pro; 15 are known to exist for a diametrically‘distorted cylinder.
vided on the shunting resistors 38 and 4-9, de?ne a corre~
Although positions other than those shown are pos
sponding corrected bridge diagonal. The other, uncor
sible for the correcting resistors,’ they should be exposed
rected, diagonal is de?ned by terminals 56 and 52. Aux
to the same environment as the active strain gauges. It
iliary bridge input and bridge output circuits 5% an‘d‘56
is especially important that temperature dependence cor
are interchangeably connectible’ across corrected diagonal
46-48 and uncorrected diagonal 5i}—52.
recting resistor 34 sense the same temperature variations
as the active portions of the strain ‘gauge bridge circuit.
The load cell housing may 'be ?lled with a silicone oil
One of the correcting resistors,'34, is for. correcting
residual bridge temperature dependence and is chosen
to eliminate temperature gradients,’ however, and the
from those resistance materials which have a very high
physical position of resistor 34 is then of minor conse-t
temperature coe?icient of resistivity, nickel or platinum, 25
for example. , The, other correcting resistor, 36, is for .
It will be realized that all of the environmentally sen
bridge ‘balance point correction and is chosen from those '
sitive bridge circuit elements are initially sealable within
materials which have a very low temperature coe?icient ' housing 24 and subjected to all signi?cant effects of the
of resistivity, constantan or manganin alloys, for exam
enclosure during the remote balancing adjustments and
I ple. Nominal impedances of correcting resistors 34 and 30 that there is no requirement for access internally of
quence.
V
.
‘ 56 are chosen to provide for a greater correction range
housing 24 during such adjustments.
"than is requiredyin a given bridge circuit. Assuming a
While there have been'descri‘bed what are at present
considered togbe the preferred embodiments of this in
vention, it will be obvious to those skilled in the art that
various changes and modi?cations may be made therein
250 ohm bridge, that is, a bridge comprised of four strain
gauges each having a nominal resistance of 25 0 ohms, the
temperature correcting resistor ‘34 may have a nominal
resistance of about .15 ohm at 75° .F. and balance point
correcting resistor 36, a nominal resistance of about .30
ohm. Approximately 2 ohms, is usually su?'icient for the
resistances of shunting resistors 38 and 40.
By selection of the position. of midtap 46, a’ controlled
7
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without departing from the invention, and it is, there
fore aimed in- the appended claims toscover all-such
changes and modi?cations as fall within the true spirit
40
and scope of the invention.
What is claimed'is:
'
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‘temperature dependence may be added to any inherent
1. A load cell comprising aspring element de?ning a
temperature dependence of the remainder of the bridge
gauged-strain'surface, ‘an hermetically sealed housing en
circuit. The sign and magnitude of the controlled tem
compassing ,said surface, and a strain gaugecircuit in
perature dependence depend upon the direction and ». cluding four substantially similar strain gauges bonded to
magnitude ‘of the displacement of ,midtap 46 from the 45 said surface and interconnected to define a four-arm
electrical center of shunting resistor. 38. Similarly, sign
bridge network, a'temperature dependence correcting re
‘ and magnitude of bridge balance point correction are
sistor connected ‘between adjacent ends of ?rst and second
determined by the displacement of midtap 48 from th
of said gauges sensing strain gauge ambient temperatures
electrical center of shunting resistor '49.
v‘ and exhibiting a variableresistance in accordance with
Final settings for the midtaps 46 and 48 maybe deter 50 said temperature, a balance point correcting resistor con
mined experimentally for each load cell by temperature
nected between adjacent ends of third and fourth of said
cycling after successive midtap adjustments. However,
gauges and exhibiting a ?xed resistance substantially in
for the mass production of similar load cells, tempera
dependent of temperature variations, ?rst and second
ture dependance vensus midtap displacement data col
' variably midtapped shunting resistorseach-connected in
lected for a prototype cell may 1be applied after but one 55 parallel Wlih'OHE of said correcting resistors, bridge in
temperature cycle, has been plottedrfor each of the fol
put and output means, oneof said means being connected
lowing models. Experimental data may also be used to
between the midtaps on said, shunting resistors, and the
predict the bridge balance point correction required for a ’ other said means being connected between adjacent ends
of said ?rst and fourth and second, and third gauges, said
any initial bridge unbalance and also the additional balance .
correction that each temperature dependence correction 60 shunting-resistors being located externally of said hous
ing, whereby load cell balance may be corrected after
will require. After temperature dependence correction
hermetic sealing of said housing.
has been accomplished, the required bridge balance point
2. The load-cell of claim 1' wherein the resistance of
correction is made by adjusting the position of midtap 43
said balance point correcting resistor is greater than that
on shunting resistor 40.
.
of said temperature dependence correcting resistor and
It is preferable that shunting'resistors' 38 and‘ 49 be of
65 the resistance of each said shunting resistor is greater
a low and stable temperature~resistivity coe?icient mate
than the resistance of the ,saidcorrecting resistor con
rial.’ However, their adjacent positions in a remote junc
’ tion box 42 obviate rigorous materials speci?cations. The
nected in parallel therewith.
resistances of" resistors 38 .and 40 should be low enough
., so thatadjustment of the midtaps 46 and 48 do not create
a 'signitic’antchangein' total bridge input and output im-‘r 70 2,680,376
Ipedances', butshould be'high enough so that resistance
variations of their lead wires are insigni?cant.
These
conditions, howevenrare satis?ed by the usual choice of
load cell lead wires and conventional resistors. wFurther,
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References Citedin the file of this patent
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2,801,388
2,801,825
_UNITED STATES PATENTS
11954
VRuge r_‘_>'_____,_>______ July. 30, 1957.
Stavnes et al. __’.._>_‘ _“__ Aug. 6, 1957
2,867,707
MacDonald __________ __ Jan. 6, 1959
2,927,292
Critchley et al.»____,____-_V_ Mar. 1, 1960
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